Conventionally, in a mobile communication system such as a PHS (Personal Handyphone System), wireless communication is established between a mobile terminal apparatus (hereinafter referred to as a terminal) and wireless base station (hereinafter referred to as a base station).
A conventional terminal typically has one antenna, and a radio frequency (RF) signal received by the antenna is subjected to various signal processing including amplification and frequency conversion in an RF receiving circuit.
One such signal processing performed in the RF receiving circuit is measurement of a reception power level (hereinafter referred to as a reception level) of the received signal. The reception level of the signal received by the antenna measured by the RF receiving circuit is displayed (notified) to the user on a display of the terminal, and hence, the user can estimate radio wave environment of the terminal.
A diversity terminal having two antennas and corresponding RF receiving circuits and selecting one of the antennas that has higher reception level as a reception antenna has been realized and disclosed, for example, in Japanese Patent Laying-Open No. 8-97759.
The diversity terminal has two antennas as mentioned above, and a received signal of one stream from either one of the antennas is selected, taken in the terminal and subjected to a signal processing for demodulation. Therefore, only the reception level of the received signal of one stream selected for demodulation is displayed for the user.
As for the base station, an adaptive array base station has been practically implemented, at which a signal from a specific user is received by a plurality of antennas, and the received signals of the plurality of streams are subjected to well-known adaptive array processing, so that the received signal from the user is separated and extracted.
Here, the adaptive array processing refers to a well-known process in which based on received signals from terminals, a weight vector consisting of reception coefficients (weights) of respective ones of the plurality of antennas of the base station is estimated and adaptively controlled, so that a signal from a specific terminal can be extracted (synthesized) accurately. As the adaptive array processing is well known, detailed description will not be given here.
As for the terminal, an adaptive array terminal having the function of adaptive array processing has also been developed. Such an adaptive array terminal has a plurality of (for example, two) antennas and corresponding plurality of (two) RF receiving circuits, and signals of a plurality of streams received by respective antennas are taken into the terminal and subjected to the adaptive array processing.
As a characteristic of the adaptive array processing, it has been known that better reception characteristic can be attained when reception level difference among the plurality of antennas is smaller. Actually, however, there is reception level difference generated among the plurality of antennas, because of difference in angle of the plurality of antennas as well as fading.
In view of the foregoing, it is desired that the reception level difference among the plurality of antennas is adjusted by some method, in order to- improve reception performance of the adaptive array terminal.
A terminal having two antennas, as is the case of the diversity terminal described above, one of which is a fixed antenna provided inside the terminal and the other is a movable antenna mounted on the outside of the terminal of which angle can be adjusted by moving itself forward or backward, has been implemented as an example of the conventional terminals.
Conventionally, however, the reception level display for the user is only the reception level of the signal of one stream received and taken in the terminal as described above, regardless of the number of antennas of the terminal.
Therefore, though the conventional terminal having a plurality of (two) antennas employs a movable antenna allowing angle adjustment of the antenna, it has been impossible to know the reception level of each of the plurality of antennas.
Therefore, similarly in the adaptive array terminal having a plurality of antennas, it has been impossible for the user to know the reception level of each antenna, and therefore, even if there is a movable antenna, it has been impossible for the user to adjust the angle of the antenna to make smaller the reception level difference among the reception antennas.
Further, in the conventional terminal having a plurality of antennas employing a movable antenna, the reception level has been adjusted by the user manually adjusting the angle of the movable antenna while visually monitoring the reception level displayed on the terminal, which adjustment has been unsatisfactory in accuracy.